Spectroscopic studies are presented of H–D isotopic exchange in the interior of icenanocrystals. The exchange process is dominated by ionic and orientational defects long viewed as governing the electrical properties of ice. A new finding that interior exchange rates can be controlled by acidic and basic adsorbates is evidence that the defects originate at the icesurface. In particular, it is argued that interior isotopic exchange is a reflection of proton concentrations equilibrated at the icesurface.